VUOT, ECMA-367, section 8.24.3

    Object Test rule

    An Object_test ot of the form {x: T} exp is valid if
    and only if it satisfies the following conditions:

        1 x does not have the same lower name as any
          feature of the enclosing class, or any formal
          argument or local variable of any enclosing
          feature or Inline_agent, or, if ot appears in the
          scope of any other Object_test, its Object-Test
          Local.

        2 T is an attached type.

        3 (Implementation restriction, 6.1 only) x does not
          have the same lower name as the Object-Test Local
          of any other Object Test of the same feature.